1. Field of the Art
The present invention relates to a process for producing a hydrophobic silica powder having an excellent re-dispersibility in organic solvents by using an aqueous silica sol as a starting material.
2. Description of the Related Art
Thus far, as a silica base material used for production of hydrophobic silica powder, precipitated silica and fine silica produced by flame thermal decomposition of chlorosilane (generally called fumed silica) have been frequently used. The methods for hydrophobic treatment include a method for hydrophobic treatment comprising bringing a silica powder into contact with a hydrophobicity-affording agent such as a surfactant or silicone oil, or a gas of silylating agent such as alkylhalogeno silane, alkylalkoxy silane, alkyldisilazane or the like, or a method for hydrophobic treatment comprising bringing a silica powder into contact with a silylating agent in a hydrophilic organic solvent containing water.
However, in any methods for hydrophobic treatment, in case where a precipitated silica or a fumed silica is used, a silica base material itself is aggregated, and thus it was impossible to obtain a hydrophobic silica powder having an excellent dispersibility.
In fact, the following hydrophobic treatments by using a precipitated silica or a fumed silica are disclosed. Any methods did not disclose a relation between primary particle diameter of a silica base material and aggregated particle diameter after hydrophobic treatment, and obtain no hydrophobic silica powder having a high dispersibility.
JP-A-2000-327321 (2000) disclosed a process for producing a hydrophobic precipitated silica comprising bringing an aqueous suspension of hydrophilic precipitated silica into contact with a catalytic amount of an acid and an organosilane compound in the presence of water-miscible organic solvent in an amount sufficient for promoting the reaction of the organic silicon compound with the hydrophilic precipitated silica.
JP-A-07-286095 (1995) disclosed a process for obtaining a silica oxide particle having a mean primary particle diameter of 5 to 50 nm, a surface treated with hexamethyldisilazane to block 40% or more of silanol groups on the particle surface, and a residual silanol group concentration of 1.5/nm2 or less.
JP-A-2000-256008 (2000) disclosed a hydrophobic fumed silica and a production process thereof characterized in that a fumed silica is subjected to a hydrophobic treatment with an organic silicon compound such as hexamethyldisilazane, and the resulting hydrophobic fumed silica has a bulk density of 80 to 300 g/l, OH group per unit surface of 0.5/nm2 or less, and aggregated particles having a particle diameter of 45 μm or more in a concentration of 2000 ppm or less.
JP-A-2002-256170 (2002) disclosed a process for producing a hydrophobic silica powder characterized by treating a fumed silica with polysiloxane and then treating with a trimethyl silylating agent.
JP-A-2004-168559 (2004) disclosed a highly dispersible, hydrophobic silica powder and a production process thereof characterized by performing a primary surface treatment with a silicone oil treatment agent, a disintegration after the primary surface treatment, and a secondary surface treatment with an alkyl silazane treatment agent after the disintegration.
On the other hand, methods are also known by using a silica sol having a high dispersibility as a starting material to make it hydrophobic. A hydrophobic silica powder is obtained by dispersing a silica sol in a hydrophilic organic solvent such as an alcohol, or in a mixed solvent of water with a hydrophilic organic solvent, reacting with a silylating agent such as an alkylhalogeno silane, an alkylalkoxy silane, an alkyl disilazane or the like and then removing the solvent. In these methods, these methods have disadvantages that a step of producing a sol dispersed in an organic solvent is complicated, the replacement or removal by distillation of the solvent is required after the reaction, and the like. In addition, the hydrophobic treatment with an alkylhalogeno silane has a disadvantage that corrosive acids are produced as by-products. Among the alkylalkoxy silanes, a monoalkoxy silane has a slightly low reactivity, a dialkoxy silane or a trialkoxy silane tends to cause a condensation reaction, and the condensation reaction often causes cross-linking reaction between particles. Therefore, it is difficult to obtain any hydrophobic silica powder having a high dispersibility. It is also difficult to remove self-condensates of alkoxy silane. Further, as an alkyl disiloxane is required to be reacted with a large amount of a mineral acid as a catalyst, this method has disadvantages that corrosive problems occur and a step for removing the acid as the catalyst from the hydrophobic silica becomes tedious. Hereinafter, examples of disclosed arts are mentioned.
JP-A-58-145614 (1983) disclosed a process for producing a silica powder having C1-36silyl groups bonded on the surface of colloidal silica in a rate of 1 to 100 per surface area 10 nm2, and homogeneous re-dispersibility in an organic solvent, comprising adding a silylating agent in an organo silica sol having a water content of 10 mass % or less, reacting each other and then distilling off the solvent.
JP-A-2000-080201 (2000) disclosed a process for producing a hydrophobic treated colloidal silica having a mean particle diameter of 4 nm or more, comprising performing a hydrophobic treatment by adding a hydrophilic colloidal silica in a mixed solvent of concentrated hydrochloric acid, isopropanol and hexamethyldisiloxane, extracting the resulting hydrophobic colloidal silica with a hydrophobic organic solvent, refluxing under heating, and then adding a silane compound, and refluxing under heating.
JP-A-2000-044226 (2000) disclosed a process for producing a surface-treated silica, comprising hydrolyzing a tetraalkoxy silane compound with a basic compound to prepare a aqueous hydrophilic fine silica dispersion and removing alcohol, then subjecting the fine silica to hydrophobic treatment with alkyltrialkoxy silane compound, replacing the solvent by a ketone solvent, triorganosilylating the reactive groups remaining on the surface of the fine silica with a silazane compound or a trialkylalkoxy silane compound, and finally distilling off the solvent under a reduced pressure.
JP-A-03-187913 (1991) disclosed a process for producing a silica powder excellent in dispersibility the surface of which is silylated, comprising reacting a methanol-dispersion silica obtained by hydrolyzing an alkyl silicate in methanol with a trimethyl silylating agent added in an amount of 5 mol % or more based on 1 mol of silica, and then distilling off excess trimethyl silylating agent and the dispersion solvent. For example, the hydrophobic silica powder is obtained by adding methoxy trimethyl silane in an amount of 20 mol % based on 1 mol of silica, to a methanol-dispersion silica obtained by hydrolyzing tetramethoxy silane in methanol in the presence of aqueous ammonia, recovering excess silylating agent and drying.